This invention relates to a method of encapsulation, in particular of encapsulating hydrophobic liquids, and to encapsulated products containing such liquids. It is concerned particularly, although not necessarily exclusively, with encapsulated products that are to be used as additions, eg. for fragrance or flavour, to a base product.
There are many known micro-encapsulation methods which employ silica particles as an encapsulating material, the function of which is to allow a delayed but continuous release of the material encapsulated. In one type, exemplified by U.S. Pat. Nos. 4440542 and 4579779, the silica particles form an open-pored matrix which holds the material, especially liquids, in its interstices. In such a structure the outer pores are exposed and the encapsulated material may not be adequately shielded from premature release, whether by chemical reaction or by leaching. Furthermore, these porous structures rely on physical sorption of the encapsulated material and have a relatively limited liquid-silica ratio, or loading. That disadvantage is more pronounced if the encapsulated material is prone to premature release since this would be countered by increasing the density of the porous structure.
These porous structures can be produced in a modified form with a substantially impervious outer shell, as exemplified in U.S. Pat. No. 4464317. Although this prevents leaching of the encapsulated material it reduces still further the loading that can be achieved. A further disadvantage of the processes described in U.S. Pat. No. 4464317 is that they produce relatively large particles (eg. &gt;500 .mu.m), of a size that is unsuitable for many processes.
In another type of process described in EP294206A, porous spheroidal silica particles are formed which contain dispersed droplets of an encapsulated liquid. It is said that a loading can be achieved of up to 50% by weight of the silica, which is not a remarkable figure, yet the dry product still has considerable porosity since, when encapsulating a perfume, it is said that any wetting produces an appreciable increase of odour. Furthermore, the preparation of the product is complex and lengthy.
In another process described in EP309054A an aqueous emulsion is formed in which droplets of lubricating oil are coated with silica particles which stabilise the emulsion. The oil is added to a partially hydrophobed silica dispersion prepared from colloidal silica and the mixture is homogenised in a high-shear mixer. In the resulting emulsion the oil droplets are held as silica-coated droplets that are stable until ruptured by pressure, as when between bearing surfaces, to produce a lubricating film.
Experiments have shown, however, that if such emulsions are formed using hydrophobic materials, such as fragrances and flavours, that do not dissolve unaided in water, these emulsions cannot be usefully employed as additives to consumer products. This is because the emulsified material can make contact with and be dissolved by other components such as surfactants in cleaning or washing products or mineral oils in cosmetic products. The emulsified material will thus simply disperse when the silica-coated droplets are mixed into the product.